Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a visible record of the electrostatic image.
In order to be useful in an electrophotographic process, a photoconductive element must display good photosensitivity and low residual voltage after exposure. Photosensitivity is a measure of the amount of energy required to discharge the photoconductor from an initial voltage to some predetermined potential. The residual voltage is a measure of the charge remaining on the element after exposing the element. The residual voltage is the minimum voltage to which a photoconductive element can be discharged. A high residual voltage can give rise to a lower potential difference between charged and discharged areas of the element on subsequent imaging cycles. Blurred, fogged, or incomplete images result. Hence, for high process efficiency, high photosensitivity and low residual voltage are desired.
Photoconductive elements, also called photoreceptors or photoconductors, are composed of a conducting support and at least one photoconductive layer which is insulating in the dark but which becomes conductive upon exposure to light. The support may be in one of many forms, for example, a drum, a web or belt, or a plate.
Numerous photoconductive materials have been described as being useful components of the photoconductive layer of a photoconductive element used in electrophotography. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines and the like. Especially useful are aggregate photoconductive compositions that have a continuous electrically insulating polymer phase containing a finely divided, particulate co-cirystalline complex of at least one pyrylium-type dye salt and at least one polymer having an alkylidenediaiylene group in a recurring unit.
Aggregate compositions used in photoconductive elements can be prepared by several techniques, such as, for example, the dye first technique described in Gramza et al., U.S. Pat. No. 3,615,396, incorporated herein by reference. Alternatively, they can be prepared by the shearing method described in Gramza, U.S. Pat. No. 3,615,415, incorporated herein by reference. This latter method involves the high speed shearing of the photoconductive composition prior to coating and thus eliminates subsequent solvent treatment, as disclosed in Light, U.S. Pat. No. 3,615,414, referred to hereinafter. By whatever method prepared, the aggregate composition is applied with a suitable liquid coating vehicle onto a support or underlying layer to form a separately identifiable multiphase aggregate composition, the heterogeneous nature of which is generally apparent when viewed under magnification, although such compositions may appear to be uniform to the naked eye. There can, of course, be macroscopic heterogeneity. Suitably, the pyrylium type dye-salt-containing aggregate in the discontinuous phase is finely divided, i.e., typically predominantly in the size range of 0.01 to 25 .mu.m.
Photoconductive elements can comprise single or multiple active layers. In a single layer photoconductive element, charge generation (the photogeneration of charge carriers, i.e. electrons and holes) and charge transport (the transportation of the generated charge carriers) take place within the same layer. Single active layer aggregate photoconductive elements are described in Light, U.S. Pat. No. 3,615,414, and in Gramza et al., U.S. Pat. Nos. 3,732,180 and 3,615,415. Single active layer aggregate photoconductive compositions have found many commercial applications.
One problem associated with photoconductive elements is a phenomenon known as dark decay. Dark decay describes the decrease in the voltage on the element between the time that it is charged by the charging device and the time that it is exposed to image-wise radiation. Dark decay reduces the potential difference between the charged and discharged areas of the photoconductive element after exposure and can result in improper placement of toner on the image. The result is blurred lines, fogging, and other undesirable artifacts in the final image. Particularly in electrophotographic processes that seek to reproduce high quality images, dark decay is a major limiting factor to preparing a useful photoconductive element.
Most known photoconductive elements display useful electrophotographic properties, including good photosensitivity, low residual voltage, and acceptable dark decay, only when subjected to one polarity (positive or negative) of charging. These are known as monopolar photoconductive elements. A monopolar photoconductive element designed for use with positive charging will have high photosensitivity when charged positively; however, the element will have little or no photosensitivity if it is charged negatively. A similar situation occurs for monopolar photoconductive elements designed for use with negative charging. This produces a limitation on the usefulness of these types of photoconductive elements in many electrophotographic processes.
In particular, electrophotographic processes that use toners of different polarities on the same image or on different images produced in the same apparatus cannot function using only one monopolar photoconductive element. For example, electrophotographic processes that use multiple layers of marking particles, known as toners, to produce color images may advantageously use toners that are charged to different polarities for each layer to produce images of higher quality. Processes that use one or more liquid toners or one or more solid toners or two or more toners of different characteristics, such as melt viscosity, surface roughness or gloss after fixing or fusing, and color or hue, may advantageously use toners of more than one polarity in an electrophotographic process. Such a process often cannot use one monopolar photoconductive element. Further, electrophotographic apparatuses that serve multiple functions, such as photocopier, printer, and facsimile machine, may advantageously use different toners with different charging polarities for each function. Again, such an apparatus often cannot use a single monopolar photoconductive element. While more than one monopolar photoconductive element can be used for such processes, this disadvantageously increases the size and complexity of the electmophotographic apparatus. Further, when two or more toners of different polarity are to be used on the same image, it is extremely difficult, when more than one photoconductive element is used, to maintain registration of the different toners on the image so that the toner is placed precisely in the collect position on the receiver. Failure to register the different toner types correctly gives rise to problems such as ghosting, where one toner is placed in slight misalignment to where it should be positioned on the receiver, and improper color reproduction. These are unacceptable for high quality images.
Single layer aggregate photoconductive compositions can be used when charged either negatively or positively. Thus, photoconductive elements containing a single active aggregate photoconductive layer are known as bipolar single layer photoconductive elements.
A problem associated with bipolar single layer photoconductive elements is that the lifetime of these elements is less than desired. Typically, the photoconductive elements are cycled repeatedly through the electrophotographic process. In each cycle, the photoconductive element is exposed to multiple charging elements, such as the primary, transfer, receiver detachment, and pre-clean erase chargers, that are extremely damaging to the photoconductive element. Exposure to charging elements frequently results in the deposition of chemical species such as nitric acid on the photoconductive element surface, causing a problem called latent image spread (LIS). In severe cases, exposure to the charging elements can also reduce the photosensitivity of the photoconductive element, ending its usefulness in the electrophotographic process. Damage of the photoconductive elements through any of these or other mechanisms caused by exposure of the photoconductive elements to charging elements will be referred to as corona-induced damage.
The lifetime of bipolar single layer photoconductive elements is also less than desired because of physical damage sustained by the elements. Physical damage to the photoconductive element inculcated during the electrophotographic process, from installation or other service procedures, or from foreign objects falling into the electrophotographic engine during normal use, can significantly reduce the lifetime of the element and will impart defects in the images produced. Such defects occur at random time intervals and cannot be treated at normal service intervals.
In order to address the issue of damage to the photoconductive element, protective layers such as sol-gel overcoats are often coated onto the photoconductive element. However, in order to be effective, the charge transport properties of such overcoats must be strictly controlled. If the material is too electrically insulating, it will not permit the photoconductive element to photodischarge when exposed to light. This will result in poor electrostatic latent image formation. Altematively, if the layer is too conducting, the electrical charges forming the electrostatic latent image will spread prior to development. This effect, referred to as latent image spread (LIS), will result in a loss of resolution and blurning of the image. It is particularly unacceptable with high quality electrophotographic engines producing latent images requiring a resolution of 600 dpi or greater. For commonly used materials such as sol-gels, the electrical conductivity is generally controlled by the addition of ionic charge conducting agents to the sol-gel formulation. However, the resistivity of such materials is highly sensitive to the humidity and can be too resistive under some conditions and too conductive under others. Further, the combination of charge conducting agents and commonly used materials such as sol-gels can frequently lead to increased susceptibility to damage from chargers rather than providing the desired protective properties. Finally, such charge agents have been found to tribocharge against commonly used toners, thereby creating image observable artifacts such as background.
It is also important that the charge conduction properties of the entire photoconductive element used in the electrophotographic process be controlled. For example, a highly resistive overcoat may be successfully used with a multi-layer photoconductive element to improve its lifetime. However, if the same overcoat is used with a single layer photoconductive element the resulting package may be too insulating to allow adequate photosensitivity. Control of the charge conduction properties of the element is particularly crucial when the element is a bipolar photoconductive element. Protective layers that are useful with monopolar photoconductive elements are designed for one specific charging polarity and may not be useful for the opposite polarity. Thus, protective layers designed for monopolar photoconductive elements may not be useful on bipolar photoconductive elements which require protective layers that are useful with both charging polarities. Therefore, it is important that the electrical properties of the overcoat layer be appropriately matched to those of the photoconductive element if the resulting photoconductive element is to function adequately in its intended use.
Changing the number of layers in a photoconductive element will alter this match in an unpredictable manner. Protective layers designed for multi-layer photoconductive elements may produce image degradation when used on single layer photoconductive elements. Thus, it is desirable to provide a protective layer that is neither too insulating nor too conductive and that is designed for the particular type of photoconductive element of interest.
Protective layers can also change the photosensitivity and residual voltage of the photoconductive element. This can result in loss of contrast between light and dark areas in the final image and in failure to reproduce some or all of an image. The impact of the protective layer on these properties depends on the combination of its properties, for example its absorption at particular wavelengths or its resistivity, with the properties of the photoconductive layers in the element. Thus, it is not obvious that a protective layer that has proven useful with one type of photoconductive element will work for all photoconductive elements.
Yet another problem with protective layers is that their adhesion to the photoconductive layers can be less than desired. Specifically, protective layers such as sol-gels tend to be rather thick (approximately 10 .mu.m). These tend to crack during use and the cracks frequently propagate through the photoconductive layers, resulting in a delamination of these layers from the support layer. Adhesive failure alone can produce image defects, and it can allow scratching or abrasion of the photoconductive element that produces image defects and decreases the element's lifetime. Adhesive failure is particularly a problem where the photoconductive element comprises a flexible support without end, also known as a web or belt. In this instance, the web must bend around rollers of various radii and is flexed severely, thereby aggravating crack formation and delamination.
Monopolar photoconductive elements having diamond-like carbon (DLC) protective layers are known. Single layer and multi-layer monopolar photoconductive elements having DLC protective layers are known. For example, U.S. Pat. No. 4,965,156 to Hotomi et al. discloses the use of two protective layers on a monopolar, organic single layer or multi-layer photoconductive element. The first protective layer is an DLC layer which includes more than 5 atomic percent fluorine. The second, outermost protective layer is a similar material except that the fluorine content must be lower than 5 atomic percent. Layer thicknesses disclosed are 0.01 to 4.0 .mu.m for the first layer and 10 to about 400 angstroms (0.00l to about 0.04 .mu.m) for the second layer. Hotomi et al. teach that if the fluorine content is above 5 atomic percent in the outermost layer, it causes image fogging. Fogging can be detected by measurements of latent image spread. This invention has several disadvantages for practical application. First, it necessitates the deposition of two protective layers of differing composition, increasing the manufacturing complexity and cost of the element. Second, the useful lifetime of the element is limited by the lifetime of the second or outermost protective layer. If the outermost layer is worn away or abraded such that any part of the first protective layer is exposed, Hotomi et al. teach that image fogging will result. Failure to deposit a defect-free second layer has the same result and is extremely likely because of the very low thickness of the layer. A method of providing protection to a photoconductive element which does not cause image fogging would be advantageous. The patent of Hotomi does not disclose bipolar photoconductive elements.
U.S. patent application Ser. No. 08/639,374 to Visser et al. discloses the use of fluorinated diamond-like carbon outermost layers on an organic, multi-layer photoconductive elements comprising charge transport layers containing arylamine. Fluorine concentrations of 25-65 atomic percent are claimed. Only multi-layer photoconductive elements are disclosed. Outermost layers of 0.05 to 0.5 .mu.m thickness are claimed. Only monopolar photoconductive elements are disclosed.
U.S. patent applications Ser. Nos. 09/023,596, now U.S. Pat. No. 4,849,445; 09/023,631 now U.S. Pat. No. 4,849,443; 09/023,896; and 09/023,901; concurrently filed on Feb. 13, 1998; to Visser et al. disclose multilayer photoconductive elements having two or more charge generation layers, at least one charge transport layer, and a diamond-like carbon protective layer with a fluorine content of between 0 and 65 atomic percent based on the composition of the protective layer. Outermost layers of 0.05 to 0.5 .mu.m thickness are claimed. Only monopolar photoconductive elements are disclosed.
U.S. Pat. No. 5,240,802 to Molaire et al. discloses the use of a silicon carbide surface layer on a bipolar photoconductive element. The photoconductive element contains a single aggregate photoconductive layer and has a surface with a 20.degree. gloss measurement value greater than about 6. While useful as a protective layer, silicon carbide is typically formed from gases such as silane (SiH.sub.4) which are difficult and dangerous to use, making them undesirable from a manufacturing standpoint. The difficulty of manufacturing silicon carbide layers also makes them expensive to use.
There is a need to provide useful bipolar single layer photoconductive elements with improved resistance to corona-induced and physical damage. In order to be considered to be a useful photoconductive element, the element must possess the following characteristics: good photosensitivity and low residual voltage with both charging polanities, low dark decay, and no latent image spread under a range of ambient humidity conditions with both charging polarities. Bipolar single layer photoconductive elements possessing these characteristics, as well as improvements in resistance to corona-induced and physical damage, would be significant improvements over the prior art.